The present invention relates to improvements in process for the production of highly concentrated phosphoric acid according to a so-called wet process involving digesting phosphate rock with sulfuric acid.
Generally, phosphate rock is treated with sulfuric acid producing solid calcium sulphate in phosphoric acid. Since in most instance, the calcium sulphate must be separated from phosphoric acid, it is essential to control reaction conditions so that the precipitated calcium sulphate is in a form amenble to separation. Depending on the conditions maintained during the reaction operation, the calcium sulphate may be precipitated as the dihydrate (CaSO.sub.4.2H.sub.2 O), hemihydrate (CaSO.sub.4 1/2H.sub.2 O) or anhydrate (CaSO.sub.4).
More particularly, the present invention is concerned with a process for the production of highly concentrated phosphoric acid and dihydrate of good quality by digesting phosphate rock with a mixed acid of sulfuric acid and phosphoric acid followed by filtration and hydrating the resulting hemihydrate, in a so-called hemihydrate-dihydrate process.
Of importance in the production of highly concentrated phosphoric acid (usually one which is 40 wt. % or more in terms of P.sub.2 O.sub.5 concentration) is to maintain the water balance in a process system in an extremely advantageous condition to obtain phosphoric acid of as high a concentration as possible and to obtain dihydrate of good quality without anhydrate crystals incorporated therein, superior in filtrability and having as small a content as possible of impurities such as phosphoric acid and fluorine compounds and so on.
Particularly, in a country whose land is too narrow to permit discard of gypsum, like Japan, great importance must be attached, in considering a phosphoric acid production process, to effective utilization of gypsum (dihydrate) by-produced in a large amount in the production of phosphoric acid [gypsum is by-produced in an amount about five times as large as product phosphoric acid (P.sub.2 O.sub.5)].
In other words, in order that the process can be utilized effectively industrially, it is necessary that not only phosphoric acid be obtained in high yield but also the byproduct gypsum be of good quality suitable to its direct use for cement setting retarder, gypsum wall board or other building materials.
As to the method of obtaining highly concentrated phosphoric acid according to a wet process, various researches and developments have heretofore been made in accordance with a hemihydrate-dihydrate process. Above all, those described in the following publications are well known: Japanese Patent Publication No. 10408/1970 (Nissan Kagaku process); Japanese Patent Publication No. 41155/1973 and Japanese Patent Laid Open No. 116110/1984 (Nippon Kokan process).
From an economic point of view, however, the above known processes have not always been satisfactory as industrial processes for obtaining highly concentrated phosphoric acid and gypsum of good quality, and the following problems have been involved therein.
For example, in the process described in Japanese Patent Publication No. 10408/1970, concentrated sulfuric acid is added in two stages in an phosphate rock digesting step in order to obtain a hemihydrate of good filtrability. More particularly, concentrated sulfuric acid is added dividedly so that 70-80% of calcium oxide in the phosphate rock is converted to hemihydrate in a first-stage digestion step and the remaining 20-30% converted to hemihydrate in a second-stage digestion step. Further, the digestion temperatures are each maintained at a temperature as high as about 90.degree. C. to about 100.degree. C., and in the second-stage digestion step the concentration of excess sulfuric acid is held in a specific range of 2% to 3%.
Thus, it is difficult and complicated to adjust the amount of sulfuric acid to be added into each digestion step. Further, according to the study of the present inventors, since the sulfuric acid added into the digestion steps is concentrated sulfuric acid, the formation of anhydrate crystals are unavoidable, which deteriorates filtrability. Besides, since the temperature of each digestion step is as high as about 90.degree.-100.degree. C., the apparatus concerned must be made of a material capable of withstanding such high temperature, e.g. Carpenter 20 (trademark), HV-9 (trademark), or Hastelloy C (trademark). But these materials are expensive.
On the other hand, in point of the P.sub.2 O.sub.5 efficiency, the amount of phosphoric acid which is lost along with by-produced gypsum cannot be ignored. More particularly, in addition to the above-mentioned necessity of obtaining gypsum of high quality capable of being directly used for cement setting retarder, gypsum wall board or other building materials, it is very important to minimize the content of P.sub.2 O.sub.5 in gypsum which is by-produced in an amount as large as 4.5 to 6 tons per ton of P.sub.2 O.sub.5. However, the gypsum obtained by the process described in Japanese Patent Publication No. 10408/1970 has a P.sub.2 O.sub.5 content (that is, phosphoric acid loss) as high as about 0.4%, which is disadvantageous in obtaining phosphoric acid in high yield. And its quality has not always been satisfactory in its use for cement setting retarder, gypsum wall board or other building materials.
Further, the processes described in Japanese Patent Publication No. 41155/1973 and Japanese Patent Laid Open No. 116110/1984 are superior in that cooling of the resulting hemihydrate slurry is not performed. In both the processes, however, reaction temperatures, which are in the range of 108.degree. to 110.degree. C., are extremely high, corresponding or close to the boiling point. Therefore, when corrosion is considered, the apparatus concerned must be made of a material capable of withstanding such high temperature, which material is more expensive than those referred to above.
Moreover, according to the processes mentioned above, since sulfuric acid is added directly into a digestion step, a partial formation of anhydrate crystals are unavoidable in the digestion step like the process described in Japanese Patent Publication No. 10408/1970. Besides, particularly in the summer season, since the temperature in hydration rises due to the increase of the outside air temperature for example, it becomes impossible to maintain the heat balance, resulting in that the sulfuric acid to be added into the digestion step must be diluted with, for example, water before its addition into the digestion step. This is inconvenient in obtaining phosphoric acid having as high a concentration as possible.
Further, in order to obtain gypsum of good quality, it has been necessary that in hydrating hemihydrate into dihydrate, a seed slurry of dihydrate crystals be prepared separately and added as seed crystals into the hydration step.